Technical Abstract:
Numerous and intricate host-parasite interactions are responsible for the successful development of insect parasitoids and the accompanying developmental arrest/demise of the host insect. Manipulation of host biochemistry, endocrinology, neurobiology and immunolgy including dramatic alterations in concentrations of hormones and neurohormones by insect endo and ectoparasitoids are the rule rather than the exception. Cotesia congregata (Hymenoptera: Braconidae) is a gregarious endoparasitoid which oviposits in young M. sexta (Lepidoptera: Sphingidae) larvae. Eggs hatch, but the parasitoids remain as 1st instars until the host becomes a last instar, at which time the parasitoids molt to the 2nd instar. Several days later parasitoid larvae concomitantly molt to the 3rd instar and emerge from the host. Parasitoid pupation occurs shortly thereafter. The last instar host ceases to feed and neither wanders nor pupates. Juvenile hormone levels are abnormally high, and although a small hemolymph ecdysteroid peak is detected, neither a wandering nor a premolt ecdysteroid peak is observed. Interestingly, based on ligation studies and prothoracic gland activity bioassays, it appears that the parasitoid interferes with host stimulatory and inhibitory mechanisms responsible for regulating host hemolymph ecdysteroid levels. In addition, immunohistochemical studies indicate that the cerebral neurosecretory cells, neurohemal organs and gut endocrine cells of host last instars contain considerably more neuropeptide than do those of unparasitized larvae, suggesting that the release of regulatory neuropeptides is inhibited.